A process of producing unsaturated aldehydes and unsaturated acids from olefins corresponds to typical catalytic vapor phase oxidation.
Generally, catalytic vapor phase oxidation is carried out by charging one or more kinds of granular catalysts into a reactor tube (catalytic tube), supplying feed gas into a reactor through a pipe, and contacting the feed gas with the catalyst in the reactor tube. Reaction heat generated during the reaction is removed by heat exchange with a heat transfer medium whose temperature is maintained at a predetermined temperature. The heat transfer medium for heat exchange is provided on the outer surface of the catalytic tube so as to perform heat transfer. The mixture containing the desired product is collected, recovered and sent to a purification step through a pipe. Since the catalytic vapor phase oxidation is a highly exothermic reaction, it is very important to control the reaction temperature in a certain range and to reduce the size of the temperature peak at a hot spot occurring in a reaction zone.
The partial oxidation of olefin uses a multimetal oxide containing molybdenum and bismuth or vanadium or a mixture thereof, as a catalyst. Typical examples thereof include a process for the production of acrolein or acrylic acid by the oxidation of propylene, a process for the production of phthalic anhydride by the partial oxidation of naphthalene or orthoxylene, and a process for the production of maleic anhydride by the partial oxidation of benzene, butylene or butadiene.
Generally, acrylic acid, a final product, is produced from propylene by a two-stage process of vapor phase catalytic partial oxidation. In a first stage, propylene is oxidized by oxygen, dilution inert gas, steam and a certain amount of a catalyst, so as to mainly produce acrolein, and in a second stage, the produced acrolein is oxidized by oxygen, inert dilution gas, steam and a certain amount of a catalyst, so as to produce acrylic acid. The catalyst used in the first stage is a Mo—Bi-based oxidation catalyst which oxidizes propylene to mainly produce acrolein. Also, some acrolein is continuously oxidized on such a catalyst to produce acrylic acid. The catalyst used in the second stage is a Mo—V-based oxidation catalyst, which oxidizes mainly acrolein in acrolein-containing gas mixture produced in the first-stage, thus mainly producing acrylic acid.
A reactor for performing such a process is provided either in such a manner that both the two-stages can be performed in one catalytic tube or in such a manner that the two stages can be performed in different catalytic tubes (see U.S. Pat. No. 4,256,783).
Meanwhile, acrylic acid manufacturers now conduct diversified efforts to improve the structure of such a reactor, or to propose the most suitable catalyst to induce oxidation, or to improve process operations, so as to increase the production of acrylic acid by the reactor.
In part of such prior efforts, propylene which is supplied into the reactor is used at high space velocity or high concentration. In this case, there are problems in that rapid oxidation in the reactor occurs, making it difficult to control the resulting reaction temperature, and also a high temperature at hot spot in the catalyst layer of the reactor and a heat accumulation around the hot spot are produced, resulting in an increase in the production of byproducts, such as carbon monoxide, carbon dioxide and acetic acid, thus reducing the yield of acrylic acid.
Furthermore, in the case of producing acrylic acid using a high space velocity and high concentration of propylene, as an abnormal increase in temperature occurs in the reactor, various problems, such as the loss of active ingredients from the catalyst layer, a reduction in the number of active sites caused by the sintering of metal components, are caused, thus deteriorating the function of the catalyst layer.
Accordingly, in the production of acrylic acid, the control of the heat of reaction in the relevant reactor is important of all things. Particularly, not only the formation of hot spots in the catalytic layer but also the accumulation of heat around the hot spot must be inhibited, and the reactor must be effectively controlled such that the hot spots do not lead to reactor runaway (a state where the reactor is not controlled or explodes by a highly exothermic reaction).
Thus, it is very important to inhibit hot spots and heat accumulation around the hot spot so as to extend the life cycle of a catalyst and inhibit side reactions, thus increasing the yield of a product such as acrylic acid. To achieve this inhibition, various attempts have been steadily made.
A fundamental method is to form several catalyst layers having activities that vary according to the moving direction of reactants (hereinafter, referred to as the “axial direction”). Namely, at a reactor inlet side where hot spots generate, a catalytic layer with low activity is formed, and catalyst layers whose activities increase slowly toward a reactor outlet side are formed. Typical methods for controlling catalytic activity include: a method of making catalytic particles by mixing a catalytic material with inactive materials (e.g., U.S. Pat. No. 3,801,634, Japanese patent No. 53-30688B, and Japanese patent No. 63-38831); a method of controlling activity and selectivity by either changing the kind of alkali metals and controlling the amount thereof (e.g., U.S. Pat. No. 6,563,000); a method of controlling activity by adjusting the occupied volume of catalytic particles (e.g., U.S. Pat. No. 5,719,318); and a method for controlling activity by controlling sintering temperature in the preparation of a catalyst (e.g., U.S. Pat. No. 6,028,220). However, such methods have some effects but still need to be improved.
Furthermore, in order to more effectively use the above-mentioned technologies, a reactor system needs to be designed such that it is suitable for oxidation with excessive heat generation. Particularly, in order to inhibit the inactivation of a catalyst caused by excessive heat generation, it is necessary to establish an efficient heat control system capable of controlling excessively high peak temperature at hot spots, thermal accumulation around the hot spot and runaway. For the establishment of the efficient heat control system, studies have been performed on the introduction of a perforated shield plate (e.g., U.S. Pat. No. 4,256,783, European patent No. 293224A, and Japanese patent No. 52-83936), the establishment of circulation pathway of molten salts by the placement of various baffles (e.g., U.S. Pat. No. 3,871,445), the design of an oxidation reactor integrated with a cooling heat exchanger (e.g., U.S. Pat. No. 3,147,084), a multi-stage heat control structure using an improved heat exchanger system (e.g., Korean patent application No. 10-2002-40043, and PCT/KR02/02074), etc.